ok, I have ceramic 9 pin minature sockets that have a small sleeve in the center, between the pins that I believe, in conjuction with the tubes cathode forms a capacitor for use in radio applications to get rid of unwanted frequencies by earthing the sleeve. My question is in audio or guitar amp applications should the sleeve be earthed or left as is. I have seen components mounted from pins to this sleeve, which believe is not good practise. Cheers
It is a convenient tie point for parts, don't over-analyze its purpose. You can run a wire to ground from it and then ground other pins to it, or run components to ground via it from the tube pins. You can use it the same way we often use pin 6 on a 6L6 socket for a terminal. The tube doesn;t use that pin, so we consider it a handy point to mount a resistor. If you wanted to put a resistor and cap in series between two pins of the socket, you could make that thing THEIR junction. Etc.
Thanks for the answers. I'm not that knowledgeable about. The old guy (now passed away) who introduced me tubes told me that was their purpose & that it wasnt good practice to mount components to them, tho I have seen a lot of with no detrimental effects. 20 odd yrs ago I built guitar amps under his guidance but havent built 1 since. The 1 I am just starting is the first in that time. Cheers
The sleeve was used for a ground point in IF and RF amplifier applications. It was tied to ground for these applications, and should be grounded if a high transconductance IF/RF pentode is used for audio. Some of these tubes have a small metal shield in the bottom which was tied to the cathode or the G3 terminal.
I doubt that it makes much difference in a typical 12AX7 type circuit, but if I was using them for the first stage in a high gain guitar preamp, I would ground the sleeve since hum and instability are always looking for ways to sneek into these kind of circuits.
Thanks 4 the info Tubelab.com. Build is JTM45 PT & rec/JTM50 power & pre amp with switchable cascading, both with 2K7/.68uf cathode, 220K plate load on 1st so its not excessively hi gain, but I've used copper lead resistors & caps for exactly the reason u mentioned so tying 2 earth cant hurt. Cheers
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